Field of the Invention
The present invention relates to an optical modulator and an optical transmission device using the optical modulator and particularly to an optical modulator having a plurality of bias electrodes and an optical transmission device using the same optical modulator.
Description of Related Art
In high-frequency/high-capacity optical fiber communication systems, optical transmission devices equipped with a waveguide-type optical modulator are often used. Among them, optical modulators in which LiNbO3 (hereinafter, also referred to as LN) having an electro-optic effect is used for the substrate allow a smaller amount of light loss and are capable of realizing optical modulation characteristics in a broader band than in modulators for which a semiconductor material such as indium phosphide (InP), silicon (Si), or gallium arsenide (GaAs) is used and thus are widely used in high-frequency/high-capacity optical fiber communication systems.
In the optical modulators in which LN is used, a Mach-Zehnder type optical waveguide, an RF electrode for applying high-frequency signals which are modulation signals to the optical waveguide, and a bias electrode for performing a variety of adjustments in order to maintain modulation characteristics favorable in the optical modulators are formed. Examples of the above-described bias electrode include a bias electrode for applying an electric field to optical waveguides in order to compensate for the fluctuation of bias points (a so-called temperature drift phenomenon) attributed to temperature changes and the like of the environment and a bias electrode for adjusting optical phases.
Meanwhile, regarding modulation methods in optical fiber communication systems, in response to the recent trend of transmission capacities being increased, multilevel modulation such as Quadrature Phase Shift Keying (QPSK) or Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) or transmission formats in which polarization multiplexing is incorporated into multilevel modulation have become mainstream.
Optical modulators performing QPSK modulation (QPSK modulators) or optical modulators performing DP-QPSK modulation (DP-QPSK modulators) include a plurality of nested Mach-Zehnder type optical waveguides and include a plurality of high-frequency signal electrodes and a plurality of bias electrodes (for example, refer to Japanese Laid-open Patent Publication No. 2010-237497), and thus there is a tendency that the size of devices increases, and there is a strong demand for, particularly, size reduction.
In the related art, as techniques for the above-described size reduction, methods in which the interaction between individual electrodes and optical waveguides is enhanced and thus the drive voltage can be reduced even in electrodes having a short length have been proposed. For example, constitutions in which a bias electrode is constituted as a comb electrode (or a blind-like electrode) that is constituted of an electrode for pushing and an electrode for pulling with respect to individual waveguides and voltages intended to be applied to bias electrodes (bias voltages) are reduced are known (for example, refer to Japanese Laid-open Patent Publication No. 2003-233042).
FIG. 21 is a view illustrating an example of the constitution of a DP-QPSK modulator of the related art. This DP-QPSK modulator 2100 is constituted of, for example, nested Mach-Zehnder type optical waveguides (heavy dotted lines in the drawing) formed on a Z-cut LN substrate 2102 and an electrode (the hatched portion in the drawing). In this optical modulator, light from a light source such as a laser diode (not illustrated) is incident on the right side in the drawing, and modulated light is emitted from the left side in the drawing. Emitted light rays are coupled using, for example, a space optical system and are incident on an optical fiber connected to a light transmission channel.
The optical waveguide is constituted of an incidence waveguide 2104 that receives incident light from the right side in the drawing, a light branching section 2106 that branches light propagating through the incidence waveguide, and two Mach-Zehnder type optical waveguides 2110a and 2110b that modulate individual light rays branched using the light branching section 2106.
A Mach-Zehnder type optical waveguide 2110a has an incidence waveguide 2112a, a light branching section 2114a that branches light propagating through the incidence waveguide, parallel waveguides 2116a and 2118a that propagate individual light rays branched using the light branching section 2114a, a Y-junction, Y-branch coupler 2120a that couples light rays propagating through the parallel waveguides 2116a and 2118a, and an emission waveguide 2122a that emits light rays coupled using the Y-junction, Y-branch coupler 2120a to the outside. In addition, the Mach-Zehnder type optical waveguide 2110a has Mach-Zehnder type optical waveguides 2130a (a portion inside the rectangle indicated by a dotted line in the drawing) and 2132a (a portion inside the rectangle indicated by a two-dot-dashed line in the drawing) that are respectively formed on part of the parallel waveguides 2116a and 2118a. 
A bias electrode 2146a which is constituted of electrodes 2142a and 2144a and a bias electrode 2152a which is constituted of electrodes 2148a and 2150a are respectively formed on the light emission side (the left side in the drawing) of parallel waveguides 2134a and 2136a of the Mach-Zehnder type optical waveguide 2130a and on the light emission side (the left side in the drawing) of parallel waveguides 2138a and 2140a of the Mach-Zehnder type optical waveguide 2132a. In addition, a bias electrode 2158a which is constituted of electrodes 2154a and 2156a is formed on the light emission side (the left side in the drawing) of the parallel waveguides 2116a and 2118a of the Mach-Zehnder type optical waveguide 2110a. 
The constitution of the Mach-Zehnder type optical waveguide 2110b is the same as the constitution of the Mach-Zehnder type optical waveguide 2110a as illustrated in the drawing. Therefore, the optical modulator 2100 includes six bias electrodes indicated by reference signs 2146a, 2152a, 2158a, 2146b, 2152b, and 2158b. In addition, in the optical modulator 2100, RF electrodes which are respectively constituted of electrodes 2170, 2172, 2174, 2176, 2178, 2180, 2182, 2184, and 2186 are also formed on eight parallel waveguides 2134a, 2136a, 2138a, 2140a, 2134b, 2136b, 2138b, and 2140b in the four Mach-Zehnder type optical waveguides 2130a, 2132a, 2130b, and 2132b. 
Here, the bias electrodes 2146a, 2152a, 2146b, and 2152b are respectively bias electrodes for adjusting the bias point of the optical modulator constituted of the Mach-Zehnder type optical waveguides 2130a, 2132a, 2130b, and 2132b, and the bias electrodes 2158a and 2158b are respectively bias electrodes for adjusting the phases of light rays emitted from the emission waveguides 2122a and 2122b. 
In addition, in the optical modulator 2100, the bias electrodes 2146a, 2152a, 2158a, 2146b, 2152b, and 2158b are constituted in a comb electrode form as illustrated in the drawing in order to reduce voltages intended to be applied to the respective bias electrodes in order to adjust the bias points or the phases.
Meanwhile, when an optical modulator is incorporated into an actually-used device and is used, it is necessary to accurately control the bias voltage so as to prevent the bias point from fluctuating in order to compensate for the above-described temperature drift and maintain light transmission characteristics in a favorable state. Therefore, to the bias electrodes for compensating for the temperature drift, low-frequency signals (dither signals) for detecting the fluctuation of the bias point and direct-current voltages (DC voltages) for compensating for the fluctuation and returning the bias point to a predetermined value are applied.
As the frequency of the dither signal, a frequency which is shorter than that of a high-frequency signal that is applied to the RF electrode and has no influence on the high-frequency signal is selected. In addition, in a case in which a plurality of bias electrodes are used, dither signals having a different frequency are used for the respective bias electrodes so as to facilitate determining which bias electrode a specific dither signal is applied to.
In this case, dither signals that are applied to the respective device electrodes are selected from a range of several kilohertz to several hundreds of megahertz in consideration of the factors of the dither signals not influencing RF signal frequencies (generally, several tens of gigahertz), frequencies not being close to each other, feedback control at a necessary velocity being possible, and the like.